The current proposal is a request for a supplement to U01 DA043908 ?Development of Novel Therapeutics for Methamphetamine Abuse? on which I serve as PI. Methamphetamine (METH) use disorder is linked to dire health and societal consequences and its use has markedly increased in the United States. Effective treatments for METH use disorder are not available. METH interacts with the vesicular monoamine transporter- 2 (VMAT2), promoting dopamine (DA) release into the cytosol and reversal of the DA transporter to increase extracellular DA, resulting in abuse liability. This project focuses on VMAT2 as a novel therapeutic target, with the overall goal of obtaining a treatment for METH use disorder. Phase 1b studies with lobeline, the initial lead, were completed; bitter taste, short half-life and the requirement for multiple daily doses were expected to reduce compliance. Lobeline was modified to obtain GZ-793A, which had the desired pharmacology, but was found to have off-target hERG interaction and potential cardiotoxicity. Iterative optimization of GZ-793A resulted in GZ-11608 and JPC-077, which were potent and selective inhibiting VMAT2 function and METH- evoked DA release in vitro. Importantly, these compounds had greatly reduced hERG activity, and demonstrated our ability to further modify the molecule to eliminate off-target interactions. These analogs also specifically decreased METH self-administration and METH-induced reinstatement in rats, at doses that did not alter food reinforcement. Tolerance did not develop upon repeated dosing. These analogs also protected against or did not exacerbate METH-induced striatal DA neurotoxicity. Despite this favorable pharmacologic profile, these analogs had low oral bioavailability (F% = 3% in rats). The current project (U01 DA043908) produced optimized GZ-11608 analogs. Again, we met the milestones and identified a very promising compound, STK-5-25, that has a Ki = 19 nM at VMAT2, no detectable hERG interaction, >4 h stability in rat microsomes and F% = 30% in rats. The current lead STK-5-25 has an aniline moiety on one of the rings, which can be associated with idiosyncratic liver toxicity with bioconversion to a reactive metabolite, although we have not observed changes in liver enzymes in rats. Our plan for the requested supplemental funds is to substitute nitrogenous heterocycles in place of the aniline as a final optimization strategy for a candidate and backup for IND enabling studies in 3 species. To do this we will employ a Contract Research Organization (CRO) to synthesize 50-100 compounds during the last few months of the active award period. Compounds will be evaluated in in vitro pharmacological assays to inform the prioritization of synthesis during the active period of the current award. Evaluation in pharmacokinetic and behavioral assays will proceed during the no cost extension period. Thus, I am requesting that NIDA provide supplemental funding, so that we can finalize the optimization of our lead compound and come back with a new GOMD application to bring us to IND submission with the goal of developing a pharmacotherapy for methamphetamine use disorder.